Mechanical Modeling And Experimental Validation Of Piezoelectric Smart Aggregates

The theoretical method, experimental investigation and practical application of structural health monitoring (SHM) as well as damage identification (DI) have received considerable attentions in academic and engineering field along with the development of civil engineering structures into large scale, complicated and intelligent technology and the emergence of new smart materials. Piezoelectric smart materials, such as Lead Zirconate Titanate (PZT), play dual functions of both sensing and actuating and have many other distinct advantages. The application of PZT provides a new way for long-term SHM and qualitative and quantitative overall damage detection technology of significant engineering structures and infrastructures. Meanwhile, the convenient and practical SHM system and damage identification strategy based on PZT have implemented the status monitoring of structure service by real-time and effective processing of the vast amounts of uncertainty data and information from monitoring system. The structural health and damage status are evaluated to ensure the safety, completeness, applicability and durability of structures. Based on the above background, the author of this thesis ambitiously engaged in the research of the mechanical modeling of PZT smart aggregate (sensor and actuator), mechanical properties and its application in SHM and damage identification. The research work and innovations concerning theoretical analysis, numerical modeling and experimental research, are as follows:Firstly, the fabrication process of piezoelectric smart aggregates has been proposed. According to the characteristics of piezoelectric smart materials and structural systems, with the concrete-based PZT as sensing and actuating element, the new multifunctional piezoelectric smart aggregates working as transducers which have good compatibility with civil engineering structures and encapsulated in concrete block, are designed and developed, respectively. The fabrication process and encapsulation technique have been further improved and perfected. The serious problem affecting its application due to bad compatibility, poor durability and vulnerability of traditional PZT materials are effectively solved. The smart SHM system and monitoring processes based on PZT smart aggregate arrays are developed in order to lay a theoretical foundation for establishment of mechanical models, analysis of mechanical properties, experimental study and its application.Secondly, the mechanical model of piezoelectric smart sensor is built, the theoretical analysis, numerical simulation and experimental validation are performed, respectively. Based on direct piezoelectric effect and PZT constitutive relation as well as the vibration principle of structure dynamics, mechanical models of surface-bonded and embedded PZT sensors are simplified and established by employing the lumped-mass method. The vibration equation of PZT sensor is solved and the voltage expression is obtained in consideration of bonding layer damping effect. Numerical simulation analysis by using harmonic exciting load is implemented to determine the correspondence relation between the mechanical and electrical properties of PZT sensors, and the influence of different parameters is further defined. Considering different adhesive thickness and applying force, the experimental study on the sensing performance of surface-bonded and embedded PZT sensor is respectively carried out using the proposed models. The theoretical model and numerical results are compared with test results to verify the correctness of sensing model.Thirdly, the mechanical model of piezoelectric smart actuator is built, and theoretical research, numerical calculation and experimental validation are performed. According to the converse piezoelectric effect and PZT constitutive relation as well as the vibration principle of structure dynamics, the PZT actuating model is developed based on the free vibration along-length direction of monolithic PZT and the surface-bonded and embedded PZT by using lumped-mass method. The motion equation of PZT actuator and the expression of actuating force are derived and solved. The relationship between input signals and output signals of PZT actuator is analyzed by numerical examples. The influence of adhesive properties and the size on the PZT actuating force is concurrently studied. In view of the effect of different adhesive properties on PZT actuating force, the proposed surface-bonded and embedded models of PZT actuator and the impact on the adhesive layer of PZT actuating force are further validated by experiments. The experimental study on the driving performance of surface-bonded and embedded PZT actuator is respectively carried out using the proposed models. The experimental results match well with the theoretical analysis and the rationality of the actuator model is verified.Fourthly, the mechanical properties of PZT smart aggregates resisting to compression and shear failure and the freeze-thaw cycle are theoretically analyzed and experimentally studied. The service performances of PZT smart aggregates are revealed. The passive monitoring technology using the PZT-based transducers is employed to detect and analyze the performance of smart aggregates under different stress and the influence of the freeze-thaw cycle on smart aggregates for the purpose of more fully and intuitively understanding their durability and mechanical properties. The results show that PZT smart aggregate has good compression, shear and freezing-thawing resisting ability, and can satisfy the requirements of the use function, which make the smart aggregates well maintain the stability of mechanical properties and ensure the same service life to concrete material in practical engineering.Finally, the decision method of damage statistical index, damage degree, damage probability and damage location of concrete structure is established and the damage statistic identification algorithm for piezoelectric smart concrete structures is proposed based on wavelet analysis. Based on the superior characteristics of PZT smart aggregate sensor and actuator embedded in the designated position of concrete structure, the real-time online monitoring experimental research is conducted and an innovative method for damage detection is demonstrated by combing the active health monitoring technology-based PZT wave with the theory of probability and statistics and wavelet analysis as well as damage detection technology. The damage index-based wavelet analysis is effectively made and damage probability and degree-based statistics characteristics are developed to evaluate the damage states according to the attenuation of propagation wave. Furthermore, the rough damage location is effectively determined. The damage index-based wavelet analysis is also performed, the damage probability-based probability and statistics are proposed to convert uncertainty of damage detection into the mathematics description in context of probability and statistic. The proposed statistical algorithm of damage identification can effectively determine damage probability and damage degree, and provide a prediction for critical damage location of concrete structure. The research findings prove the tremendous effectiveness and feasibility of structural health comprehensive monitoring and damage detection technique in various large-scale concrete structures identification utilizing PZT transducers.